Silicon-On-Insulator (SOI) based technology allows a micro-electronic or Micro-Electro-Mechanical System (MEMS) device to be fabricated in a silicon layer that is located above an insulating layer (e.g. a buried oxide layer). The insulating layer is located over a silicon substrate. Electronic devices, such as transistors and MEMS-type devices are fabricated in the layer of silicon located on top of the insulating layer. This technique may provide higher speeds and use less power by reducing capacitance, reducing or eliminating the reverse leakage of the p-n junctions and thus making device operation in SOI superior to devices fabricated in conventional Complementary Metal-Oxide Semiconductor (CMOS) bulk silicon based processing.
One MEMS type device that may be implemented in SOI is a pressure sensor. Pressure sensors typically include a piezo-resistor coupled with a diaphragm. The piezo-resistor is embedded in the diaphragm, and responds to a change in stress of the diaphragm with a change in resistance as a consequence of the piezo-resistive effect. When the pressure applied to the diaphragm changes, the amount of deflection of the diaphragm changes accordingly, which results in a change in the stress level in the silicon diaphragm. This in turn causes the piezo-resistor element to increase or decrease in resistance. Thus, the increase or decrease in resistance may be used to gauge the amount of pressure being applied to the diaphragm.
Pressure sensors are used in a wide variety of environments. Some environments include high temperatures and/or high pressures. Because the pressure sensor is fabricated from semiconductor materials that have different thermal coefficients, extreme temperatures may cause the various layers of the pressure sensors to expand at different rates. In particular, the silicon dioxide (SiO2) electrical isolation layer expands and contracts at a different rate than the silicon layer that comprises the piezo-resistor.
As a pressure sensor is cycled between low and high temperatures, the electrical isolation layer may begin to crack. This is especially true if stress concentration areas are present. Cracking may also be caused by extremely high pressures or the combined effects of high temperature and pressure. The present application describes a way to minimize stress concentration areas in piezo-resistive, SOI pressure sensors and other MEMS devices.